<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.5.17/dist/vue.js"></script>
</head>
<body>
    <div id="app">
        <div v-if="isShow">
            <span>用户名:</span>
            <input type="text" key="username">
        </div>
        <div v-else>
            <span>手机号:</span>
            <input type="text" key="mobile">
        </div>
        <button @click="isShow=!isShow">切换</button>
    </div>
</body>
<script>
    const app = new Vue({
        el: '#app',
        data: {
            input: '',
            isShow: true
        }
    })
</script>
</html>